Just to add to the good other answers and to pick up on a few things you said in the comments (and sorry, can't comment, so also mentioning a few things in reply to comments right here):
- I'll concur in the advice to keep things simple, even more so as you mentioned the BCO already has all the details. What you may need to be prepared to add (again in simple terms), where it doesn't pass as obvious by itself, is to give the simple explanation about the details, the bigger picture about what you are really doing.
Which, as others have pointed out, should not sound like a person unreliable about where they live or about political issues - but none of these seem to really fit your case as what you are really doing seems quite simple and straightforward (and legal) so you should be perfectly fine.
- Most importantly, you are core mainland EU citizen (obviously, having a German passport) traveling with the intent to set up (if rather temporarily) in another EU country.
(Note that "to set up" is an imprecise term for a good reason - within the EU, you are not required to know your own plans any more precisely than that as - for border control considerations, you're practically only moving in-country.)
*No idea why you are making a transfer inside the EU through the US but whatever. (Curious to know, though.)
Not only that's the most legal thing for you to do in your position, it is legal to the extent that you don't even need any kind of paperwork to do it - you just go and that's it. (FWIW it's legal to the extent that you don't even have any paperwork about it simple because none is required. Other than the German idea of registering your place of residence, which is largely really about municipal taxes and urban development.)
 Formally, for those not familiar with it, it's called the Schengen Area and using that is roughly the same as going from one US state to another US state, inside the US (obviously), is for an US citizen.
which is a) perfectly legal, b) perfectly understandable if one is so inclined, c) should be perfectly meh to an INS officer and the like who would be looking for possible violations of immigration laws - something you neither intend nor would have any connection with your own plans.
Consistently with the above, on a side note,
- a "titre de sejour" is exactly what the OP would never need anywhere inside the EU, already being a Schengen Area citizen and that of the core countries such as Germany, France, Italy, Belgium, Spain etc. (@NicolasFormichella - no offense but quite confused that (being in Paris and looking Italian, no offense again) you wouldn't know that.) A "titre de sejour" is a permit for EU aliens (non-citizens of the EU) asking to be legally living (for a usually limited, though often practically indeterminate given period of time) in the issuing country, other than on whatever visa.
On some other side notes,
So it seems you have been around the world a lot, and obviously always within the legal (and never trying to stay in the US in any awkward way). No red flags to US immigration visible whatsoever.
What's a problem about world travel anyway? Australians, for one example, do it a lot, and so what? Can't be unfamiliar to US BCOs.
might could be your thinking is being a bit overly anal (as in detail-possessed) and getting caught up in that about what to say? If a form is asking information in a simplistic way, it's not necessarily that the form is insufficient. Well maybe it is, but making you enter information in a crudely simplifying manner seems to suggest that doing so is considered sufficient for its purpose.
As for the form entry specifically, when there is no such thing as "last" or "intended" country of residence, it might seem natural to think "natural country of residence" as in "citizenship"
Your German citizenship should preempt your residency for most requirements from the comments above
This. I guess that, if asked, would be something that could have legitimately seemed obvious to you even if for some reason you were not sure about the form. I mean, if the form isn't asking something very specific (sorry and no offense about the form), what else could be the natural thing to enter?
You are temporarily not registered in Germany as living there on a municipal register level (which is arguably not even a category to a BCO I guess so no point in even bringing it up), so?
- Seems more interesting what would happen if you were stranded anywhere in the world and that country expelled you, where would you end up being? Being a German citizen, that would be Germany. End of question.
It seems unlikely to me that you would be confronted about entering the "wrong" choice in the form. If it did happened, it would be plausible (as is the case) that you were confused about details that are probably not even relevant (guess the BCO would know what should have been entered to give them the information they need?) your confusion could innocently come across as momentarily sheepish, but obviously innocent.
Seems to me (though not a thing to state) that restrictive forms to enter things in can be thought of as morons, either that or entities that just don't care beyond the point of what you can enter. Also, if obviously no one there cares about more exact information (not having made it possible to even enter more exact information), it must be good enough to enter the information you can enter. "That form did not allow me to specify more, so I entered as much as it asked me to. That's what it asked, so meh."
On a closing note, I feel curious to hear how it went - would you feel like following up with us?